1. Field of the Invention
This invention relates to a system and method for calibrating the exit thermocouples used in a pressurized water reactor in connection with generating online three-dimensional power distributions within the reactor core.
2. Background Information
A pressurized water reactor has a large number of elongated fuel assemblies mounted within an upright reactor vessel. Pressurized coolant is circulated through the fuel assemblies to absorb heat generated by nuclear reactions in fissionable fuel contained in the assemblies. An ex-core detector system mounted outside of the reactor vessel provides a measure of the average power generated by the fuel assemblies. However, it is also important to know the distribution of power through the core to assure that operating limits are not exceeded. The power distribution is affected by a number of factors, such as for instance, the degree of insertion of control rods into the fuel assemblies.
Systems have been developed to determine the power distribution in a pressurized water reactor. One system known as BEACON consists of a set of coupled, yet independent, computer software programs, which execute concurrently on one or more engineering workstations to generate on-line three-dimensional power distributions in the reactor core. The BEACON system uses an incore flux map together with a three-dimensional analysis to yield a continuously measured three-dimensional power distribution. The functions performed by BEACON include core monitoring and core analysis including predictive functions such as on-line shutdown margin evaluations, estimated critical condition calculations and load maneuver simulation.
The flux maps are generated by running moveable detectors through instrumentation thimbles in some, but not all, of the fuel assemblies. The data generated is processed to produce a map of core power distribution. While such flux maps provide the most accurate core power distributions, they are only performed during startup and at intervals typically not more frequent than once a month during operation of the reactor. It takes several hours to complete data collection for one flux map and the reactor needs to be at a specified steady state condition during this period. Also, these detectors would be quickly depleted if used continuously. As a result, the flux maps are employed to calibrate the three-dimensional analytical nodal model.
In order to maintain the accuracy of the three-dimensional power distribution information between flux mappings, other instrumentation has been utilized for measuring power distribution within the core. The power developed in individual fuel assemblies can be determined by the change in enthalpy of the coolant as it passes through the assembly. Enthalpy, in turn, is a function of the temperature rise over the fuel assembly, the pressure of the coolant and certain properties of the coolant. The coolant pressure remains fairly constant, but in any event, is a measured quantity, and the properties of the coolant are known. The rise in temperature is measured by inlet temperature sensors which measure the temperature of the coolant as it circulates back to the reactor core. Average inlet coolant temperature to the fuel assemblies can be measured accurately. Some but not all, of the fuel assemblies are fitted with exit thermocouples. The enthalpy change in the instrumented assemblies can be calculated by measuring the temperature rise over the fuel assembly. If the coolant flow rate of the assembly is accurately known, then the power produced in the assembly is accurately obtained. The fuel assembly in a pressurized water reactor does not have an enclosure channel which prevents the coolant from cross flowing among the neighboring assemblies.
The effect of the cross flow is characterized by the mixing factor which is defined as the ratio of the measured assembly power and the power determined from the measured enthalpy rise by the thermocouple. These mixing factors depend on the thermocouple location in the core and the reactor power level. These measured mixing factors are used to update the three-dimensional analytical nodal model. Power distribution uncertainties are evaluated by generating a standard deviation of the mixing factors from each thermocouple. These uncertainties are applied by the BEACON system to the measured power results.
The thermocouples can vary substantially in performance and need to be calibrated. Prior to this invention, thermocouple mixing factors and mixing factor standard deviation have been obtained using data collected mostly at full power during the previous fuel cycle. The mixing factors have been calculated as the ratio between a measured power determined from a periodically generated flux map, and the enthalpy rise determined by the thermocouples. These mixing factors have been power independent coefficients for each thermocouple. Most importantly, when a fuel cycle is completed, and the reactor is refueled and serviced, changes to the fuel assembly distribution as loaded in the core and the thermocouple instrumentation results in inappropriate mixing factors for the next cycle.
There is a need therefore for an improved method and system for calibrating the exit thermocouples used in monitoring systems for pressurized water reactors.
There is a particular need for such a method and apparatus which can determine the quality or uncertainty of the thermocouple mixing factors for conditions of the current fuel cycle rather than relying on data from a past cycle.
These needs, and others, are satisfied by the invention which is directed to a method for calibrating the exit thermocouples provided in a pressurized water reactor. Temperatures measured by each of the thermocouples are repetitively recorded during power ascension in the reactor. As used throughout, power ascension means an increase of power of at least about 50% and includes initial power ascension from startup, as well as subsequent power increases of at least about 50%. Calibration factors and associated statistics are generated for each thermocouple from the measured assembly power during the power ascension as a function of core average power. These correction factors are applied to correct subsequent measurements of temperature taken by the thermocouples. Specifically, the calibration factors generated are the mixing factors. The statistics are used to calculate the standard deviations of the mixing factors and the related quality or uncertainty of the associated thermocouple data. These uncertainties are applied to the measured power results from the thermocouples.
For each temperature measurement for each thermocouple recorded during the power ascension, a corresponding predicted power for the thermocouple at the time of the temperature measurement is stored with the temperature measurement along with the core average power. It is preferred that the measurements of thermocouple temperature be recorded for discrete ranges of power during ascension. The number of measurements stored for each power range is limited, so that should the power level be leveled off for any reason during power ascension, the data is not distorted. The predicted power is generated from a three-dimensional analytical nodal model of the reactor core. The mixing factors are generated by converting the measured temperature into a thermocouple power value and dividing this thermocouple power value into the corresponding predicted power. The power dependent mixing factors for each of the thermocouples are fitted to a selected mixing factor function of reactor power. Standard deviations of the fitted to actual mixing factors are generated for each thermocouple. All of the standard deviations are then fitted to a single function of assembly power.
Periodically, the selected mixing factor function of reactor power is adjusted. This adjustment is carried out by generating a flux map of the reactor core using moveable detectors. This flux map is used to update the three-dimensional analytical nodal model to generate a reference measured power distribution. A thermocouple measured temperature for each thermocouple taken at the same conditions at which the flux map was generated is converted to a reference thermocouple power. An associated reference mixing factor is then generated for each thermocouple by dividing the reference measured power for the thermocouple determined from the reference measured power distribution by the reference thermocouple power. The selected mixing factor function for each thermocouple is then adjusted to pass through the associated reference mixing factor at the core power. Such periodic adjustment can be carried out multiple times during initial power ascension. It can also be performed at full power and then periodically after initial calibration whenever a flux map is generated.